In wireless communications systems, such as LTE (Long Term Evolution) standardized in the 3GPP (3rd Generation Partnership Project), it is assumed that a plurality of wireless base stations are disposed, where each wireless base station communicates with wireless terminals (referred to as terminals hereinbelow) within its communication area. The communication area is referred to as a cell, which may be divided into a plurality of sub-regions by imparting directivity to an antenna. The sub-region is referred to as a sector cell. In the following description, the term cell refers to a sector cell.
In LTE downlink transmission, a terminal measures communication channel quality information such as an SINR (Signal to Interference plus Noise Ratio), converts it into a quantized CQI (Channel Quality Indicator) referring to a lookup table or the like, and reports the result to the base station. The lookup table is generally created by a link-level simulation in which a physical layer is simulated.
When transmitting data to the terminal, the base station uses the reported CQI to calculate an estimated SINR (SINR_Est), and selects an MCS (Modulation and Coding Schemes) Index referring to a lookup table. Again, the lookup table is generally created by a link-level simulation in which a physical layer is simulated. Then, a TBS (Transport Block Size) Index is determined referring to a lookup table (NPL 1) from the MCS Index, whereby data can be transmitted with optimum data size.
SINR_Est is calculated from EQ. (a) below. SINR_CQI represents an SINR corresponding to a CQI, and Offset represents an offset.SINR_Est=SINR_CQI+Offset  EQ. (a)Control for regulating Offset above is OLLA (Outer Loop Link Adaptation). In case that a notification of a reception error (NACK) is received from a terminal, Offset is updated to a smaller value, as given by EQ. (b) below; and in case that a notification of a successful reception (ACK) is received, Offset is updated to a greater value, as given by EQ. (c) below, whereby the SINR_Est can be corrected to achieve a target reception error rate. Delta_Down represents an arbitrary fixed value, and T_Bler represents a target reception error rate.On reception of NACK: Offset=Offset−Delta_Down  EQ. (b)On reception of ACK: Offset=Offset+T_Bler/(1−T_Bler)*Delta_Down  EQ. (c)
On the other hand, as measures for addressing an increase of traffic in recent years, as shown in FIG. 12, attention is attracted to a heterogeneous network in which cells of various sizes are present together by introducing a base station with low transmission power (small cell base station) in addition to an ordinary macro base station; however, since the same wireless band is used between adjacent cells, inter-cell interference is perceived as a problem. It is an interference management technology that circumvents the problem. For an interference management technology according to 3GPP Release 10, an eICIC (enhanced Inter-Cell Interference Coordination) is standardized and an ABS (Almost Blank Subframe) is configured (NPL 2). Here, the eICIC is also referred to as a time domain ICIC. The ABS is also referred to as a Protected Subframe. A base station having the ABS configured stops transmission of a data channel (PDSCH: Physical Data Shared Channel) in a downlink. Thus, an SINR in a terminal in an adjacent cell is significantly improved, and enhancement of the throughput of that terminal can be expected.
With introduction of the ABS, a base station can define two measurement subframe sets differentiated between the ABS and a non-ABS from Release 10 (NPL 3). A terminal measures communication channel quality information for each measurement subframe set, and reports a CQI of each set. Thus, a base station can calculate the SINR_Est with high precision regardless whether the subframe is an ABS or a Non-ABS. In an example shown in FIG. 13, four subframes (#1, #3, #5, #9) in a macro cell is defined as an ABS, and a pico terminal connected to a pico cell has Measurement Subframes 1 and 2 configured according to the ABS and the Non-ABS, respectively.